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June 4, 2005. Prions, those infectious proteins responsible for scrapie, mad cow, and human diseases like Creutzfeld-Jacob, are usually deposited in the brain as non-amyloidogenic, highly toxic aggregates. But a report in yesterday’s Science shows that by simply removing a lipid moiety that anchors prions to the cell membrane, the proteins are instead deposited as amyloid-like plaques, reminiscent of those found in Alzheimer disease (AD). What’s more, without the lipid anchor, mammalian prion is rendered much less infectious and much less toxic.

The glycosylphosphatidylinositol (GPI) lipid anchor has previously been shown to reduce conversion of normal cellular prion protein (PrP) into the protease-resistant variety of the protein, often abbreviated as PrPSc (see Kocisko et al., 1994). This prompted Bruce Chesebro at the National Institute of Allergy and Infectious Diseases, Hamilton, Montana, together with colleagues there and at Scripps Research Institute and the University of San Diego, both in La Jolla, California, to delve deeper into the role of the lipid anchor. They generated transgenic mice lacking the signal sequence necessary for GPI attachment to cellular prion. Then they challenged the transgenic animals with various strains of infectious scrapie prions. What Chesebro and colleagues first noted was that the transgenic mice were completely devoid of clinical symptoms. Whereas all wild-type mice developed clinical signs of disease within 150 days of infection, the mice harboring the GPI-negative prion remained disease-free for up to 600 days, at which point all the wild-type animals were dead. Over the course of the 600 days, only two out of over 100 infected transgenic mice fell ill, but with no obvious signs of neurologic disease, these deaths appeared unrelated to scrapie infection.

If the animals had no clinical signs of disease, then perhaps the loss of the GPI anchor completely protected PrP from conversion to the toxic, protease-resistant PrPSc. To test this idea, Chesebro and colleagues took protein samples from the infected transgenic animals and measured their ability to reinfect normal mice. This experiment showed that the infected transgenic mice did harbor contagious prions, though at titers much lower (about tenfold lower) than found in infected wild-type animals. The results indicate that the loss of the GPI anchor somehow reduced prion infectivity.

But what was happening to the prion that was converted into infectious agent? Even if only 10 percent was turned into PrPSc, shouldn’t the animals show some clinical signs of disease? To find out what was happening to the GPI-negative prion, Chesebro and colleagues searched for PrPSc in the transfected transgenic mice. The authors found that PrPSc did indeed appear in the animals’ brains about 200 days after being infected with scrapie, and that the amount of the protease-resistant prion continued to increase as the transgenic animals aged. Remarkably, some of the transgenic animals had even more PrPSc than was found in infected wild-type animals. So why were the animals with GPI-negative prion not falling ill?

When Chesebro and colleagues examined histological samples, they found that the brains of the transgenic mice were full of dense, plaque-like PrPSc deposits. These were unlike the diffuse deposits seen in wild-type animals. The plaques, found in the cerebral cortex, hippocampus, hypothalamus, forebrain, and around blood vessels, were very similar to those plaques found in Alzheimer brains. The deposits stained readily with thioflavin S, for example, a dye that has high affinity for deposits formed from a variety of amyloidogenic proteins.

Overall, the findings indicate two major roles for the GPI anchor. It makes the prion more infectious and it prevents PrPSc from being deposited as amyloid—the two properties may well be connected. Exactly how the GPI prevents prion from forming amyloid is unclear, but the authors suggest that it could be related to membrane attachment or inhibition by the GPI moiety itself. Wild-type prion contains much more carbohydrate than the GPI-negative prion, for example, and these sugars might interfere with the refolding necessary for amyloid formation.

The implications of these findings for prion biology seem profound. Writing in a Science perspective, Adriano Aguzzi, University Hospital of Zurich, Switzerland, suggests that it is now “almost unavoidable to conclude that prion replication avails itself of membrane-bound signal transducers to elicit brain damage.” By disengaging PrPc from the cell surface, Chesebro and colleagues have “effectively uncoupled clinical disease from prion replication,” he writes. As such, the finding might lead to new therapeutic strategies for dealing with prion toxicity.

The findings also support a widely accepted hypothesis about Aβ toxicity in AD, namely, that the large amyloid plaques are not the most toxic form of the protein; instead, that honor falls to smaller, Aβ oligomers. Given that the GPI-negative animals show no obvious clinical symptoms, the findings also support the idea that large aggregates, or inclusion bodies, may well be protective, a theory that has been debated for some time.—Tom Fagan

This study adds fuel and new information to the debate on whether it is fibrillar amyloid or oligomerized species of amyloidogenic proteins that are the neurotoxic moities in neurodegenerative brain amyloidoses such as prion diseases, AD, PD, and other tauopathies and synucleinopathies.

Prion disorders include Creutzfeldt Jakob disease in humans, bovine spongiform encephalopathy in cattle, and chronic wasting disease in the deer and elk population of North America. The underlying pathogenic and infectious particle in all prion disorders is the scrapie prion protein that arises from a change in conformation of the host-encoded prion protein to the pathogenic, scrapie form. Deposits of scrapie prion protein in the brain parenchyma have been considered hallmarks of prion disorders, until the recent report by Chesebro and colleagues that provides a new perspective on this much-debated question.

By infecting transgenic mice expressing a soluble form of the prion protein, the authors demonstrate accumulation of scrapie plaques without clinical symptoms of prion disease. Although accumulated plaques resemble those observed in Alzheimer disease, lack of associated pathology provides a new perspective on the pathophysiology of prion disorders, and by extension, Alzheimer disease. This study demonstrates that, contrary to current belief, the plaques may in fact be neuroprotective, sequestering the more toxic molecules into an innocuous clump. This concept, though not totally novel, will change the therapeutic strategies that are currently aimed at reducing the plaque load in both prion and Alzheimer diseases.